In general, electroluminescent elements used in electroluminescent displays and electroluminescent lighting apparatuses emit light on the basis of the principle of recombination of positive holes injected from an anode and electrons injected from a cathode in an electroluminescent layer and subsequent excitation of the luminescence center caused by the recombination energy.
FIG. 2 is a schematic sectional view of a known general electroluminescent element, wherein an electrode (cathode) 1, an electroluminescent layer 2, a transparent electrode layer (anode) 3A and a translucent body (transparent substrate) are sequentially laminated.
In electroluminescent displays, it is preferable that the light generated in the electroluminescent layer is emitted with a high efficiency. However, the light emitted at an angle close to the critical angle included in the generated light is totally reflected by the interface between the light-emitting surface, i.e., the transparent substrate, and air, and thus becomes guided light (substrate mode) that travels inside the transparent substrate in the planar direction while being totally reflected. There is also other guided light (thin film mode), which travels inside the transparent electrode layer or the transparent electrode and electroluminescent layers in the planar direction while being totally reflected by the interface between the transparent electrode layer and the transparent substrate. These kinds of guided light are attenuated due to absorption in the element and thus can not be output.
The presence of these kinds of guided light has lowered the efficiency with which light is emitted from the transparent substrate of known electroluminescent elements (ratio of the amount of light emitted from the electroluminescent element to the total amount of light generated in the electroluminescent layer) to as low as approximately 20%.
Patent Document 1 discloses an organic electroluminescent apparatus that has a light-scattering portion formed of a lens sheet in its transparent substrate or has a matte substrate, as a technique for reducing the amount of these kinds of guided light. However, although this technique can reduce the amount of the guided light traveling inside the translucent body in the planar direction while being totally reflected by the interface between the translucent body and air, the other guided light, which travels inside the transparent electrode layer or the transparent electrode and electroluminescent layers in the planar direction while being totally reflected by the interface between the transparent electrode layer and the transparent substrate, can not be output, thus reducing the light emission efficiency.
Patent Document 2 discloses an information display wherein frustrating elements are provided in order to eliminate the internal total reflection of light. In this technique the frustrating elements that contain bulky diffusing material and are placed at several sites within the device can slightly improve the light emission efficiency, but potential contact between the textured interface and the transparent electrode would cause a lot of problematic dark spots, which may also shorten the life of the element, to form on the light-emitting surface. In addition, the guided light that travels inside the transparent electrode layer or the transparent electrode and electroluminescent layers in the planar direction while being totally reflected by the interface between the transparent electrode layer and the transparent substrate can not be output, thus reducing the light emission efficiency, just like the technique described in Patent Document 1.    Patent Document 1: Japanese Patent No. 2931211    Patent Document 2: PCT Japanese Translation Patent Publication No. 2004-513483